1. Field of the Invention
The invention relates to a process for operating a calender with a roll stack which features two end rolls and in between several middle rolls that rest against one another in a press direction, with at least one roll having an elastic surface. The invention further relates to a calender with a roll stack which features two end rolls and in between several middle rolls, with at least one roll having an elastic surface.
2. Discussion of Background Information
Such calenders are used in particular for glazing paper or cardboard webs. The invention is described below on the basis of the treatment of a paper web. However, it can be used in the same way with other material webs with which similar problems occur.
When glazing a paper web, the paper web is guided through the calender and into nips that are formed between a hard and a soft roll, i.e., a roll with an elastic surface, and is acted upon by increased pressure and, if necessary, also by increased temperature. In calenders of more recent construction type, e.g., the “Janus” calenders, rolls are used that are covered with a plastic coating. It can now be observed that in many cases crosswise stripes occur on the paper web after a certain operating time. As soon as these stripes become visible, the paper web is unusable and forms waste. The reasons for this so-called barring formation has not yet been conclusively determined. However, it is assumed that they are the effects of a vibration phenomenon. However, vibrations are virtually unavoidable in a calender.
In the barring formation, the elastic surface of the soft roll is changed. It has not yet been conclusively determined what this change actually entails. The following possibilities are currently assumed: the rolls develop a waviness on the surface, i.e., a hill and valley structure, that the roll becomes polygonal, or that the roll alternately develops zones of differing surface quality, e.g., differing roughness, in the circumferential direction. Regardless of the particular type of change, after the barring formation, periodic stripes appear in the axial direction on the circumference of the roll. Corresponding stripes then appear on the paper web, whereby the paper web is to be considered as waste at the latest by the time the stripes become visible.
When a barring phenomenon occurs, the roll that causes the barring formation has to be removed, reground or finished. The service life of such a roll is therefore limited.